300 is above the renal threshold, so yes, staying above 300 will cause organ/tissue damage long term. And glucose toxicity, which will increase insulin resistance. If you're cat is staying above 300, your vet should realize that the insulin resistance is due to glucose toxicity. If not overcome, then yeah, you "have to live with it". But your cat will have a shorter life.
Here is some good reading, and maybe share it with your vet.
http://www.felinediabetes.com/FDMB/threads/feline-diabetes-and-glucose-toxicity-links.62960/
I'd also mention that if you don't believe "Every Cat Is Different", as a fact or as an answer to some of your questions, then sorry, you're not going to find lots of answers to your questions. Your cat is different than every other cat that any of us have ever had with diabetes. If they were all the same, then either every one would go into remission like mine did, or none of then would ever go into remission. What worked for my cat worked for my cat. I've never seen any other cat in the five years I've been here that what treated the same way as my cat was, and chances are, if they had been they would not have gotten the same results.
You can work around the dry food. It will probably require higher doses of insulin than you would need if your cat conformed to a low carb wet diet. But not all cats do or will. Great that you are home testing. If you don't use a Spreadsheet, I at least hope you are keeping a log of your own so that you can see "history". I never had a SS either, just kept a hand-written log. It will be nearly impossible to get a handle on things if you don't at least do that. People here will have a hard time helping you deal with this logically with no spreadsheet, because that is the way all of us are "wired". As long as you have a good record of his numbers, then at least you have something to look at to try to make sense of numbers, doses, etc. But you'll pretty much be flying solo if you have questions about dosage, unless you can use your vet for that.
Personally, in your shoes, I would edit my signature. Screaming in ALL CAPS at people wanting to help you figure this out is probably not going to find you much in the way of help. Just my opinion.